Mobile wireless communications systems generally require the capability of handovers between base station transceivers. However, such handovers have traditionally created technical issues resulting in dropped calls during the handover process.
One such issue with respect to GSM networks is that of maintaining control channel integrity when using low-rate Advanced Multi-rate Codec (AMR) modes, half-rate traffic channel modes, or indeed for many other logical traffic channels. For example, when operating using the full-rate AMR 5.9 bps or 4.74 kbps speech encoder modes, as well as half-rate or other modes as mentioned above, the carrier-to-interference plus noise ratio (CINR) required to maintain an acceptable frame error rate (FER) on the traffic channel (TCH) may be significantly less than the CINR required to maintain the control channel FER. Example control channels include, but are not limited to, the Slow Associated Control Channel (SACCH) and the Fast Associated Control Channel (FACCH) as specified in the GSM 3GPP specifications, among other various control channels.
Because control channel reception is critical for certain operations such as handovers, the control channel error rate is of particular importance for reducing the network dropped call rate (DCR).
It would be desirable to combine multiple control channel transmissions by for example Chase combining. However, if any bit or bits change between subsequent transmissions, the codeword resulting from forward error correction (FEC) coding methods, such as the combination of Fire coding and convolutional coding in the case of the GSM FACCH, would also change and direct combining of control channel blocks would not be possible.
One potential solution, which could be applied to the FACCH, would be to permit re-transmission of an identical message, thereby permitting the Mobile Station (MS) to combine the first and second transmissions. However, such a method presents several additional issues.
First, the method requires that the 184-bit payload (Layer 2 or “L2” message) of the first and second FACCH transmissions be exactly identical to permit Chase combining at the receiver. No modification whatsoever of the FACCH message content could be permitted.
Second, in order to permit the receiver to combine the appropriate control messages, some means of implicit or explicit signaling would need to be provided to instruct that combining should be performed. For example, the time between the first and second transmission of the FACCH frame could be an exact number, or otherwise known number, of TDMA frames.
Third, the method could only be used to combine a known limited number of FACCH transmissions. Flexible support for the Chase combining of many FACCH transmissions would prove impractical. Finally, the method could not be supported by legacy networks or provide significant advantage to legacy terminals.